The particle tracking detector for the ALPHA-g experiment. A unique radial TPC that will be used in a measurement of the effect of gravity on antihydrogen.
The size of the ALPHAg apparatus led ALPHA to choose a different annihilation detection technology from ALPHA 2. Using a radial Time Projection Chamber (rTPC) we can construct a very long, and uniform detector. The ‘radial’ in the name denotes the direction of the electric field.
TPC on a crane to be installed into the experiment
Just like the Silicon Vertex Detector, we need to encourage electrons to keep on moving once they have been nudged by a passing charged particle. Instead of doing this in a solid (like silicon) we use a gas. With a high voltage across the gas, any electrons that have been nudged, will separate from the atoms they are orbiting and accelerate, knocking more electrons as they travel. This cascade of electrons gives us a pulse of charge on the outside wall which we can record.
Shows a cross section of part of the detector, the green line shows a pion (one of the possible products of an antihydrogen annihilation), the orange lines show the path that the generated electrons follow, the grey dots a cross section of the wires we use to accelerate electrons in the ‘drift region’ (the detector volume), then the black wire we use to read out the signal (the current of electrons). The Read-out pads at the back collect charge generated from the pion between the anode wires and the readout pads. The delayed arrival of the longer orange traces allows us to reconstruct the entire track.
Since the annihilation products interact with the entire gas volume, many points can be reconstructed, giving us a great accuracy in determining the track. See some reconstructed tracks of a real annihilation below:
- For more about Antimatter in general, have a look at the CERN Antimatter page
- Good descriptions of 'How ALPHA works' in detail can be found in the publication section on our website
- Particularly appropriate for students and non-physicists are Cold Antihydrogen: a new frontier in fundamental physics, Antihydrogen for precision tests in physics and Antihydrogen in a bottle
- For experts, a general overview of many of the important aspects can be found in Search for Trapped Antihydrogen